Screen stencils are generally formed by first coating the porous support with a radiation, or light sensitive, composition, then protecting the image areas with a positive, and subsequently exposing the screen to sufficient light to cure the nonimage (unprotected) areas. The screen is then washed to remove the uncured composition, leaving unblocked image areas. The cured, nonimage areas remain coated, and can be considered blocked areas. The blocked and unblocked areas together form a printing stencil.
The most common method of preparing a screen stencil, the conventional direct process, requires coating the porous support, such as a framed fabric, with a fluid composition, such as a solution, emulsion, dispersion, or the like, which contains the light-sensitive material. The carrier for such compositions is a liquid, such as water. The carrier is then removed by drying, leaving behind a coating of solid, light-sensitive, i.e., light-curable, material on the fabric. The coated fabric is then brought into intimate contact with a positive by sandwiching it and the positive between a glass support and a rubber sheet, in an airtight assembly, and drawing a vacuum. The intimacy of contact between positive and fabric necessary for good resolution upon printing, however, requires the use of a strenuous condition of reduced air pressure. Such conditions would, of course, be unsuitable for use with a light-curable material that is itself liquid at ambient room temperature, due to the evolution of dissolved or entrained gases, such as air, with resultant bubble formation.
This direct process is limited to the use of a light-curable material that can be rinsed away when dried but not cured, but is insensitive to the rinsing medium if cured after drying. The hydrocolloid materials in general use for this process, however, often remain sensitive to water rinsing, or moisture in the air, or the printing vehicle or ink solvent, after light-curing. Any sensitivity of the cured coating to its environment of use reduces the resolution in printing provided by the screen stencil, and often causes breakdown of the stencil.
Further, when a typical hydrocolloid material is initially coated onto a fabric, it is generally coated in excess, and the excess is scraped off, leaving uniformity of coating thickness to the skill of the stencil maker, or to chance. If less than excess amounts of material are applied to the fabric, areas of the fabric may be insufficiently coated, and durability as well as uniformity of thickness again cannot be assured. Lack of uniformity of coating adversely affects deposit resolution and can result in at least portions of the print being excessively thick.
Excessively thick coatings on screen stencils are objectionable, particularly if a stencil is to be used with ultra-violet curable inks. The thickness of the screen stencil determines, in part, the thickness of the ink deposit. Ultra-violet curable inks dry by curing, not by solvent evaporation, and an excessively thick ink deposit can be too opaque, blocking radiation from curing the ink through to the substrate.
High quality, fine detail screen printing is also generally recognized in the field as requiring that the edges of the nonimage areas (the break between nonimage and image areas) present smooth lines or curves, without serrations or "saw tooth" when viewed either vertically or horizontally. To achieve this, in part, the coating must be formed so as to present a planar surface to a substrate to be printed, and be uninfluenced by the cross points of, and open spaces between, the fabric filaments. Generally, such a planar surface cannot be formed when using conventional direct process coating materials wherein the light-curable coating is dried, by evaporation, from a liquid vehicle that may comprise about two-thirds of the coating material, because substantial shrinkage ensues. The coating tends to align itself within the openings, coating the filament cross points at a higher plane. Multiple coatings generally do not substantially alleviate this lack of planar surface of the coating.
Another conventional, but less commonly used, method of preparing a screen stencil utilizes a solid film of light-curable hydrocolloid that is exposed to light through a positive before it is applied to the porous support (stencil blank). The hydrocolloid film is supported on a transparent support sheet during exposure, and during the subsequent washing to remove uncured portions. The washing wets and softens the cured portions, and thereby allows the cured portions to become attached to a stencil blank when pressed down over the remaining areas of the hydrocolloid film. It is generally recognized that the hydrocolloid film is only partially cured on the side facing away from the light source, and this partial curing may enhance the attachment of the hydrocolloid film to the stencil blank. When the exposed and washed hydrocolloid film dries, the support sheet is removed, leaving it secured to the stencil blank.
This process provides a reasonably planar surface to present to a substrate to be printed. Some of the partially cured material opposite this planar surface is generally washed away, reducing coating thickness. The reduced coating thickness, together with the largely mechanical bond between coating and stencil blank, tend to make screen stencils produced by this process very fragile. Multifilament fabrics must be used to achieve even modest durability. These screen stencils, having been prepared with hydrocolloid materials, are also moisture and solvent sensitive.
In a "direct-indirect" process, a preformed, unsensitized, hydrocolloid film is transferred to a stencil blank, and treated to sensitize it and secure attachment to the stencil blank. It is thereafter subjected to the exposure and washing steps of the direct process. This process does not ensure the formation of a planar surface; the hydrocolloid film tends to sink into the openings between filaments.
The use of a light-curable material that is itself liquid at ambient room temperature is known in the art, for example the process and materials disclosed in the British Pat. No. 618,181. Therein is described impregnating and/or coating the screen material with a carrier-free photosensitive material, superimposing a positive (referred to therein as negative) on the screen, and exposing through the positive to light. No provision is made for ensuring intimate contact between either positive and/or the positive, fabric, and any intermediary film, or uniformity of coating thickness, or complete encapsulation of fabric filaments, all deemed necessary for the printing resolution demanded by the field today.